Fischertechnik Dynamic Magnetic Chain Ball Carrier
This is an extra ball carrier that goes onto the motorized chain of the Fischertechnic Dynamic XXL marble run set. We were a bit disappointed to find only 4 such carriers in the set, so we made some more.
It holds a standard 4x4x4mm cube magnet in the center, that you can just press fit in. You should be able to find those on ebay or banggood.
Print in STL orientation, i.e. with the clips pointing horizontal. That will yield the most strength, as the part clips onto the chain, just like the originals.